Electrolytic cells containing an anode, a cathode, and a solvent-containing electrolyte are known in the art. One trend in battery technology is toward nonaqueous lithium electrochemical cells. Such cells typically included an anode of metallic lithium, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents, and a cathode of an electrochemical active material, typically a chalcogenide of a transition metal. More recently, insertion compounds have replaced metallic lithium in the anode.
These solid, secondary battery typically comprises several solid, secondary electrolytic cells in which the current from each of the cells is accumulated by a conventional current collector so that the total current generated by the battery is roughly the sum of the current generated from each of the individual electrolytic cells employed in the battery.
During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode, where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active cathode material through the electrolyte and are plated back onto the lithium anode.
U.S. Pat. No. 5,456,000, which is incorporated by reference in its entirety, discloses the formation of electrolytic cell electrodes and electrolyte film/separator elements. The electrodes and electrolyte film/separator elements use a combination of a poly(vinylidene fluoride) copolymer matrix and a compatible organic solvent plasticizer to provide battery component layers, each in the form of a flexible, self-supporting film.
An electrolytic cell precursor, such as a rechargeable battery cell precursor, is constructed by means of the lamination of electrode and electrolyte film cell elements which are individually prepared. Each of the electrodes and the electrolyte film/separator is formed individually, for example by coating, extrusion, or otherwise, from compositions including the copolymer materials and a plasticizer. The materials are then laminated, as shown in FIG. 1.
In the construction of a lithium-ion battery, for example, a copper grid may comprise the anodic current collector 110. An anode (negative electrode) membrane 112 is formed by providing an anodic material dispersed in a copolymer matrix. For example, the anodic material and the copolymer matrix can be provided in a carrier liquid, which is then volatilized to provide the dried anode membrane 112. The anode membrane 112 is positioned adjacent the anodic current collector 110.
An electrolyte film/separator membrane 114 is formed as a sheet of a copolymeric matrix solution and a plasticizer solvent. The electrolyte film/separator membrane 114 is placed adjacent the anode membrane 112.
A cathode (positive electrode) membrane 116 is similarly formed by providing a cathodic material dispersed in a copolymer matrix. For example, the cathodic material and the copolymer matrix can be provided in a carrier liquid, which is then volatilized to provide the dried cathode membrane 116. The cathode membrane 116 is then overlaid upon the electrolyte film/separator membrane layer 114, and a cathodic current collector 118 is laid upon the cathode membrane.
The assembly is then heated under pressure to provide heat-fused bonding between the plasticized copolymer matrix components and the collector grids. A unitary flexible battery precursor structure is thus produced.
An extraction process of the prior art is graphically represented in FIGS. 2a-2c. During processing of the battery precursor 220, a large quantity of a homogeneously distributed compatible organic solvent plasticizer 222 is present in the solid polymeric matrix, as represented in FIG. 2a. Prior to activation of the battery, however, the organic solvent 222 is removed, as represented in FIG. 2b. This is generally accomplished using an extracting solvent (not shown) such as diethyl ether or hexane, which selectively extracts the plasticizer without significantly affecting the copolymer matrix. This produces a "dry" battery precursor 224 substantially free of plasticizer and which does not include any electrolyte solvent or salt. As represented in FIG. 2c, an electrolyte solvent and electrolyte salt solution 226 is imbibed into the "dry" battery copolymer membrane structure to yield a functional battery system 228.
This prior art extraction requires the use of hazardous chemicals, such as ether or hexane, to produce the "dry" electrolytic cell precursor, which is then activated by introduction of electrolyte solution (electrolyte solvent and electrolyte salt) to form the electrolytic cell.
An alternate prior art method of forming an electrolytic cell, i.e., a displacement method, is graphically represented in FIGS. 3a and 3b. As represented in FIG. 3a, a large quantity of a homogeneously distributed plasticizer 322 is present in the solid polymer matrix of the battery precursor 320. Rather than extracting this plasticizer 322, as discussed above, the plasticizer is instead displaced by an electrolyte solution 326. Displacement of the plasticizer 322 by the electrolyte solution 326 is stated to be virtually complete, as represented in FIG. 3b.
However, it has been found that complete removal of the plasticizer is not obtained using either the extraction or the displacement methods of the prior art. In a preferred embodiment of the prior art, using tetrahydrofuran (THF) as the plasticizer, it is necessary to completely remove all traces of the plasticizer. THF has relatively poor electrochemical stability, such that the presence of even minor amounts of THF or similar electrochemically unstable components can "poison" the battery, causing poor cycling of the battery and a shortened battery life.
In view of the above shortcomings associated with the prior art, there is a need for solid state electrochemical devices that are capable of providing improved manufacturing parameters, and improved battery performance.